The Organic Light Emitting Display (OLED) possesses many outstanding properties of self-illumination, low driving voltage, high luminescence efficiency, short response time, high clarity and contrast, near 180° view angle, wide range of working temperature, applicability of flexible display and large scale full color display. The OLED is considered as the most potential display device.
The OLED can be categorized into two major types according to the driving methods, which are the Passive Matrix OLED (PMOLED) and the Active Matrix OLED (AMOLED), i.e. two types of the direct addressing and the Thin Film Transistor (TFT) matrix addressing. The AMOLED comprises pixels arranged in array and belongs to active display type, which has high lighting efficiency and is generally utilized for the large scale display devices of high resolution.
The AMOLED is a current driving element. When the electrical current flows through the organic light emitting diode, the organic light emitting diode emits light, and the brightness is determined according to the current flowing through the organic light emitting diode itself. Most of the present Integrated Circuits (IC) only transmit voltage signals. Therefore, the AMOLED pixel driving circuit needs to accomplish the task of converting the voltage signals into the current signals. The traditional AMOLED pixel driving circuit generally is 2T1C, which is a structure comprising two thin film transistors and one capacitor to convert the voltage into the current.
A 2T1C pixel driving circuit traditionally employed for AMOLED, comprises a first thin film transistor, a second thin film transistor and a capacitor. The first thin film transistor is a switch thin film transistor, and the second thin film transistor is a drive thin film transistor, and the capacitor is a storage capacitor. Specifically, a gate of the first thin film transistor is electrically coupled to a scan signal, a source is electrically coupled to a data signal, a drain is electrically coupled to a gate of the second thin film transistor and one end of the capacitor; a drain of the second thin film transistor is electrically coupled to a power source positive voltage, and a source is electrically coupled to an anode of the organic light emitting diode; a cathode of the organic light emitting diode is electrically coupled to a power source negative voltage; the one end of the capacitor is electrically coupled to the drain of the first thin film transistor and the gate of the second thin film transistor, and the other end of the capacitor is electrically coupled to the source of the second thin film transistor and the power source negative voltage. When the AMOLED shows images, the scan signal controls the first thin film transistor to be activated, the data signal enters into the gate of the second thin film transistor and the capacitor, and then the first thin film transistor is deactivated. Due to the storage function of the capacitor, the gate voltage of the second thin film transistor can still maintain the data signal voltage so that the second thin film transistor is in the activated state. The driving current enters the organic light emitting diode through the second thin film transistor to drive the organic light emitting diode to emit light.
The 2T1C pixel driving circuit traditionally employed for the AMOLED is highly sensitive to the threshold voltage of the thin film transistor, the channel mobility, the trigger voltage and the quantum efficiency of the organic light emitting diode and the transient of the power supply. The threshold voltage of the second thin film transistor, i.e. the drive thin film transistor (particularly as the driving thin film transistor is a low temperature polysilicon thin film transistor) will drift along with the working times. Thus, it results in that the luminescence of the organic light emitting diode D is unstable; furthermore, the drifts of the second thin film transistors of the respective pixels, i.e. the drive thin film transistors are different, of which the drift values may be increasing or decreasing to cause the nonuniform luminescence and uneven brightness among the respective pixels. The traditional 2T1C pixel driving circuit without compensation can causes 50% nonuniform brightness or even higher.
One method to solve the nonuniform AMOLED display brightness is to add a compensation circuit to each of the pixels. The compensation means that the compensation has to be implemented to the parameters of the drive thin film transistor, such as threshold voltage or mobility to each of the pixels to make the current flowing through the organic light emitting diode irrelevant with these parameters.